1. Field of the Invention
The present invention is directed to a computed tomography system equipped with a data-acquisition system, as well as a method for use with such a computed tomography system.
2. Description of the Prior Art
A computed tomography system of the type noted above has an X-ray source that directs a pyramidal X-ray beam through an object that is to be examined, e.g. a patient, onto a radiation detector. The X-ray source and, in some types of computed tomography systems, the radiation detector as well, are mounted on a gantry that can be rotated about the patient. The patient can be lying on a table that may be displaced or moved relative to the gantry along a system axis. This, inter alia, makes it possible to scan a portion of the patient""s body in a spiral manner, so that a volume of the patient""s body scanned. The values measured during the process are used to reconstruct sectional views of planar slices of the patient.
The radiation detector of the computed tomography system can have a linear detector array composed of a row of several detector elements aligned adjacent to one another or, as described, for example, in U.S. Pat. No. 5,291,402, several parallel linear detector arrays. The advantage of employing a computed tomography system equipped with a multi-line radiation detector is, for example, the ability to more rapidly scan a volume of the patient""s body that is of interest, which, among other things, reduces the recording time. The disadvantage of employing a computed tomography system equipped with a multi-line radiation detector, however, is that it has a larger number of detector elements, which means that more detector elements must be read out during each scanning step than in the case of a computed tomography system equipped with a single-line radiation detector. This, in turn, leads to higher data rates for signals coming from a data-acquisition system that reads out the detector elements.
A radiological therapy device that has a photodiode array preceded by an X-ray image intensifier is known from U.S. Pat. No. 5,117,445. The photodiode array preceded by an X-ray image intensifier converts X-ray radiation into electrical signals. The electrical signals may be falsified, however, by geometric non-linearities of the X-ray image intensifier, which is why at least some of the output signals at the photodiode array must be interpolated in a suitable manner.
An object of the present invention to provide a computed tomography system of the type initially described with a data-acquisition system configured so that a prerequisite for lower data rates is satisfied. Another object of the invention is to specify a method for use with such a computed tomography system that will allow the data rates to be reduced.
The first object is achieved in accordance with invention in a computed tomography system equipped with a data-acquisition system and a radiation detector that has at least one linear detector array composed of several detector elements aligned up adjacent to one another, the data-acquisition system reading out the detector elements and forming and further processing difference signals from signals read out from pairs of adjacent detector elements. Since it is highly likely that any differences in the amplitudes of signals read out from pairs of adjacent detector elements will be slight, the rates of change of their difference signals will also be slight. Employing difference signals will thus allow data rates to be reduced compared to the conventional case where signals read out from single detector elements are employed. In a variant of the invention the data-acquisition system has at least one analog/digital converter that digitizes the signals read out from the detector elements. This advantageously makes it possible to form the difference signals at the digital level.
In an embodiment of the invention the data-acquisition system has at least one analog/digital converter that digitizes the difference signals read out from pairs of adjacent detector elements.
If, in an embodiment of the invention, the digitized difference signals have a data length of one byte, the data rates for a computed tomography system according to the invention will be low.
In a further embodiment of the invention the radiation detector is formed of several detector modules each composed of several linear detector arrays, which in turn each have several detector elements aligned adjacent to one another, and each of the detector modules is assigned an analog/digital converter.
The other object of the invention is achieved in a first embodiment of a method for acquiring signals from a computed tomography system equipped with a data-acquisition system and a radiation detector that has at least one detector module with at least one linear detector array with several detector elements aligned adjacent to one another, having the following method steps:
a.) reading out each detector element of a detector module at each scanning step and
b.) forming signals S*j,m,k from signals read out from detector elements of the jth detector module, where
S*j,m,1=Sj,m,1 for k=1
and
S*j,m,k=Sj,m,kxe2x88x92Sj,m,kxe2x88x921 for 1 less than kxe2x89xa6K,
where Sj,m,k is the signal read out from the kth detector element of the mth linear detector array of the jth detector module and K is the number of detector elements in the mth linear detector array.
As explained above, it is highly likely that signals read out from pairs of adjacent detector elements will have slight differences in amplitude. Since the rates of change of the difference signals will thus also be slight, employing the difference signals will allow data rates to be reduced compared to the conventional case where signals read out from single detector elements are employed.
According to the invention, the second object also is achieved in a second embodiment of a method for acquiring signals from a computed tomography system equipped with a data-acquisition system and a radiation detector that has at least one detector module with at least one detector column with several detector elements aligned adjacent to one another, having the following method steps:
a.) reading out each detector element of a detector module at each scanning step and
b.) forming signals S*j,m,k from signals read out from detector elements of the jth detector module, where
S*j,1,k=Sj,1,k for m=1
and
Sj,m,k=Sj,m,kxe2x88x92Sj,mxe2x88x921,k for 1 less than mxe2x89xa6M,
where Sj,m,k is the signal read out from the mth detector element of the kthdetector column of the jth detector module and M is the number of detector elements in the kth detector column.
The reduced rates of change of the difference signals may thus be exploited in an advantageous manner for reducing the data rates.
In a variant of the invention, the signals Sj,m,k are further processed in digital form in the method. It is thus advantageously possible to form the difference signals at the digital level.
In an embodiment of the invention, the signals S*j,m,k are further processed in digital form in the method.
If, in a further embodiment of the method, the digital difference signals have a data length of one byte, data rates may be kept low in an advantageous manner.